3 Answers
3

In searching the official rules forum for Savage Worlds I've found two relevant posts. They are basically in line with what @SevenSidedDie outlined, although there is a slight implication that if magic requires hands at all an offhand penalty would be appropriate:

Casting in Melee - Basically a question about whether or not it is allowed to cast spells in melee and if their are penalties. By core rules, it is allowed, but possibly not with certain trappings. (Trappings are the setting specific rules for magic, like using hands or spell components)

Off Hand Magic - Also relevant, in this case specifically addressing the offhand penalty. Again, it depends on trappings, but the admin does point out that offhand penalties can apply to any action.

In closing, it depends on your trappings. However, if hands are required for spellcasting there is probably an offhand penalty and if hands are not required there probably isn't.

The rules don't cover details like this, leaving them up to the fiction of the "rules of magic" in your setting.

Consider things like "wizards (do/don't) need both hands to cast (most) spells" to be a trapping that applies to the entire Arcane Background in your setting. You can either work this out as the GM beforehand or as it comes up, or you can invest some setting authority in your players and ask them, working out together how magic works. (If you do turn it around and ask your players, remind them that enemy wizards will be using the same rules of magic—there's no "best" answer to how magic works, since the advantages and disadvantages can be exploited by everyone.)